The present invention generally relates to driving circuits, and more particularly to a laser diode driving circuit.
A semiconductor laser which is made up of a laser diode is extremely compact and is capable of carrying out a direct modulation by a driving current at a high speed. For this reason, the semiconductor laser is popularly used as a light source of optical disk units, laser printers and the like.
However, the optioal output characteristic of the semiconductor laser with respect to the driving current changes greatly depending on the temperature. Hence, the temperature characteristic of the semiconductor laser becomes a big problem when setting the optical output of the semiconductor laser to a desired value. Various automatic power control (APC) circuits have been proposed to eliminate this problem and bring out the advantageous feature of the semiconductor laser. The previously proposed APC circuits ca roughly be categorized into the following three systems.
According to a first system, a light receiving element is provided to monitor the optical output of the semiconductor laser. The optical output of the semiconductor laser is controlled by forming a photoelectric negative feedback loop which constantly controls a forward current of the semiconductor laser so that a signal proportional to a photocurrent generated in the light receiving element becomes equal to a light emission level instruction signal.
According to a second system, a light receiving element is provided to monitor the optical output of the semiconductor laser. A forward current of the semiconductor laser is controlled so that a signal proportional to a photocurrent generated in the light receiving element during a predetermined power setting period becomes equal to a light emission level instruction signal. The forward current of the semiconductor laser which is set during this predetermined power setting period is stored during another period. During the predetermined power setting period, the optical output of the semiconductor laser is modulated by modulating the forward current of the semiconductor laser with reference to the stored forward current during a time in which information is added to the optical output of the semiconductor laser.
According to a third system, the temperature of the semiconductor laser is measured. The optical output of the semiconductor laser is controlled by controlling a forward current of the semiconductor laser depending on the measured temperature o by controlling the temperature of the semiconductor laser constant.
Out of the above described systems, the first system is preferable in order to set the optical output of the semiconductor laser to a desired value. However, the first system suffers from a problem in that the system cannot operate at a high speed.
As an improvement of the first system, a laser diode driving circuit was proposed in a Japanese Laid-Open Patent Application No. 63-114285. According to this proposed laser diode driving circuit, a light receiving element is provided to monitor the optical output of the semiconductor laser. The optical output of the semiconductor laser is controlled by forming a photoelectric negative feedback loop which drives a base of a bipolar transistor by a current which corresponds to a difference between a photocurrent generated in the light receiving element and a reference current.
However, according to this proposed laser diode driving circuit, a phase delay is introduced due to the frequency characteristic of the current amplification of the bipolar transistor. For example, when the current amplification of the bipolar transistor is 100 and the transition frequency is 5 GHz, a phase delay of 45.degree. is introduced at approximately 50 MHz. For this reason, there are problems in that the frequency at which the loop gain of the photoelectric negative feedback loop becomes "1" cannot be set to a high frequency, and the optical output of the semiconductor laser cannot be controlled at a high speed.